A suicide bomber has targeted a national security building in Syria's capital Damascus causing serious injuries, Syrian TV says. Ministers and security officials were meeting inside at the time and some of those attending were critically hurt. The area around the building, in Rawda district, has been sealed off. The attack comes amid claims of a major rebel offensive on the city. Syrian officials said security forces had fought off "terrorist attacks". Meanwhile, UN chiefs have been trying to persuade China and Russia to agree tougher measures on Syria, ahead of a Security Council vote on Wednesday on imposing sanctions. UN-Arab League envoy Kofi Annan held talks with Russian President Vladimir Putin in Moscow, and UN Secretary General Ban Ki-moon met Chinese leader Hu Jintao in Beijing. The UN has until Friday to renew the mandate for observers in Syria, although a vote is expected in New York on Wednesday afternoon. Western nations want a new resolution threatening measures short of the use of force. The Western-backed draft resolution to be discussed gives the Syrian government 10 days to withdraw heavy weapons from cities and return troops to barracks, otherwise a further resolution on sanctions will be submitted to the Security Council. But the BBC's Jim Muir in neighbouring Lebanon says that with Russia resisting all efforts to persuade it to take a tougher line with Syria, there is virtually no hope of concerted international action to pull the country back from the brink. In other developments: * Turkish officials report two Syrian generals are among hundreds of refugees who fled into Turkey overnight, bringing the total number of fleeing generals to 20 * Turkish President Recep Tayyip Erdogan is expected to discuss the Syrian crisis with Russian President Vladimir Putin * Iraq warns its citizens to flee the violence, hours after the bodies of two killed journalists were handed over by the Syrian authorities 'Damascus volcano' Witnesses at the site of the bombing said the area had been cordoned off and journalists were banned from approaching. "The terrorist explosion which targeted the national security building in Damascus occurred during a meeting of ministers and a number of heads of [security] agencies," the TV said. Earlier activists reported more clashes during the night in several areas around the south-west of Damascus. They said the government had brought more troops and armour into some districts, and that several people had been killed in clashes and bombardments. A rebel spokeswoman, Susan Ahmad, told the BBC the entrances to Damascus were closed in the morning. "We heard the sounds of explosions every now and then all around Damascus," she said. "Now tanks are storming into al-Qaboun [district], shelling everything, shelling residential houses, shooting every moving thing and they are trying to arrest people and kill. "People are trying to run away and get out of al-Qaboun." Activists have also posted on the internet pictures of what they say is a barracks on the heights overlooking the city engulfed in flames. They believed it had been hit by fire from Free Syrian Army (FSA) rebels, and said the barracks is involved in providing security for the presidential palace complex below. State media said security forces fought off attacks by small groups of armed terrorists in the city. But the TV carried night-time footage of troops deployed in the Midan quarter, in some very tense and deserted streets. The rebels have declared a final battle for the capital, calling it Operation Damascus Volcano, and have been fighting troops in several parts of Damascus for the past three days. The fighting reached central areas on Tuesday, with gunfire and plumes of smoke reported in a street near parliament. The Free Syrian Army said the operation was well planned, and they had sent hundreds of fighters to the capital last week to be in place for the assault. The rebels and the government often publish contradictory accounts of the same incidents. Western journalists are under heavy restrictions in Syria, making it difficult to verify the claims of either side. About 16,000 people have died in Syria since protests against the Assad regime began in March last year.